Conventional processes for the preparation of water soluble cationic polymers which are useful as flocculants for waste water treatment and as papermaking chemical additives include aqueous solution polymerization, water-in-oil emulsion polymerization, and suspension or bead polymerization in hydrophobic solvent.
Other methods include the preparation of water soluble, nonionic or anionic polymers by precipitation polymerization in an aqueous solution of ammonium sulfate. Still others have carried out the polymerization of water soluble monomers in an aqueous solution of salt in the presence of a polyhydric alcohol or a polyelectrolyte as a dispersant.
Takeda et al. U.S. Pat. No. 4,929,655, which issued on May 29, 1990, the disclosure of which is hereinafter incorporated by reference into this specification, provides a novel process for preparing a water soluble polymer dispersion which overcomes the many disadvantages of solution polymerization, water-in-oil emulsion polymerization and suspension polymerization processes. This process for the production of a water soluble polymer dispersion includes the polymerization of water soluble monomers including 5 to 100 mole % of a cationic monomer represented by formula (I) below, 0 to 50 mole % of another cationic monomer represented by formula (II) below and 0 to 95 mole % of (meth)acrylamide in the presence of 1 to 10% by weight (based on the total weight of monomers) of an organic high molecular weight multivalent cation dispersant containing at least one monomer of formula (II) below (i.e., a hydrophilic monomer), in an aqueous multivalent anionic salt solution having a concentration of 15% by weight or more. Cationic monomers represented by Formula I have the structural formula: ##STR2## where R.sub.1 is either hydrogen or CH.sub.3 ; R.sub.2 and R.sub.3 are each an alkyl group having 1 to 3 carbon atoms; A.sub.1 is either an oxygen atom or NH; B.sub.1 is either an alkylene group having 2 to 4 carbon atoms or a hydroxypropylene group, and X.sub.1.sup.- is an anionic counterion.
Cationic monomers represented by formula (II) have the chemical structure: ##STR3## where R.sub.4 is either hydrogen or CH.sub.3 ; R.sub.5 and R.sub.6 are each an alkyl group having 1 to 2 carbon atoms; R.sub.7 is a hydrogen atom or an alkyl group having 1 to 2 carbon atoms; A.sub.2 is either an oxygen atom or NH; B.sub.2 is either an alkylene group having 2 to 4 carbon atoms or a hydroxypropylene group and X.sub.2.sup.- is an anionic counterion.
Takeda et al. U.S. Pat. No. 5,006,590, which issued on Apr. 9, 1991, the disclosure of which is hereinafter incorporated by reference into the specification, is similar to Takeda '655, except that it teaches the use of a dual dispersant system wherein polymerization is carded out in the presence of: (1) a water soluble cationic polymer which is insoluble in an aqueous solution of a polyvalent anion salt; and (2) a water soluble cationic polymer which is soluble in an aqueous solution of a polyvalent anion salt. The water soluble cationic polymer that is insoluble in the aqueous solution of polyvalent anion salt contains at least 5 mole % of cationic monomer units represented by the aforementioned formula (I) above and the water soluble cationic polymer that is soluble in the aqueous solution of a polyvalent anion salt contains at least 20 mole % of cationic monomer units represented by formula (II) above.
Although the final polymer dispersion viscosities are satisfactory, i.e., 1,000 cps or below, the processes disclosed in U.S. Pat. Nos. 4,929,655 and 5,006,590 both proceed through very high process viscosities (i.e., &gt;100,000 cps), which require the use of a specially designed high viscosity polymerization reactor. Due to these high process viscosities, the methods disclosed in U.S. Pat. Nos. 4,929,655 and 5,006,590 can only be used in the synthesis of polymer dispersions containing relatively low polymer content (i.e., 15 to 20%) by weight.
That is, in the dispersion polymerization processes discussed above, a polymer, for example, an acrylamide (AcAm)/dimethylaminoethyl-acrylate benzyl chloride quaternary (DMAEA.BCQ)/dimethyl-aminoethylacrylate methyl chloride quaternary (DMAEA.MCQ) terpolymer particle is adsorbed or grafted by dispersants such as polydimethylaminoethylacrylate, methylchloride quaternary (polyDMAEA.MCQ) and colloidal stabilization is attained. During this process the hydrodynamic volume of the particle, consisting of water, salt, monomers, macro radical, and dispersant, is significantly increased. This increase is presumably due to the hydrophilic nature of polydimethylaminoethylacrylate methyl chloride quaternary (DMAEA.MCQ) which is highly extended in the polymerization medium. The increase in hydrodynamic volume results in a relative increase in the bulk viscosity.
We have discovered that if the dispersion polymerization is carried out in the presence of a hydrophobically associating dispersant comprising a copolymer of diallyldimethylammonium chloride and a surface active monomer selected from the group of monomers having the formula: ##STR4## where R.sub.1 is either hydrogen or CH.sub.3 ; R.sub.2 and R.sub.3 are each an alkyl group having 1 to 3 carbon atoms; A.sub.1 is either an oxygen atom or NH; B.sub.1 is either an alkylene group having 2 to 4 carbon atoms or a hydroxypropylene group, Q is benzyl, or C.sub.6 -C.sub.20 alkyl, and X.sub.1.sup.- is an anionic counterion and alkyl esters of acrylic acid process viscosity during polymerization may be maintained below about 10,000 cps permitting the use of conventional reactors.
The present invention also provides many additional advantages which shall become apparent as described below.